Motor vehicles usually have one or several exhaust-gas sensors disposed in the exhaust duct, which emit a measuring signal that is proportional to a concentration of at least one exhaust-gas component and allows its concentration to be determined. For instance, Lambda sensors supply a measuring signal that provides information about the oxygen concentration in the exhaust gas and thus about the air-fuel ratio supplied to the internal combustion engine, and NOx sensors supply a signal that corresponds to the concentration of nitrogen oxides NOx. Most of these sensors require a specific operating temperature for reliable measuring accuracy, which is why they generally have an internal sensor heating device, which heats the sensor to its operating temperature, especially after a cold engine start.
It is conventional in this context that the ceramic elements normally used in the sensors exhibit a very sensitive response to the entry of condensate, especially liquid water, which may lead to damage and malfunction of the sensor. After a cold engine start, these sensors are therefore heated to their operating temperature only if it is ensured, by corresponding heating of the exhaust system, that condensate precipitation is no longer able to occur and act on the sensor at the installation location of the sensor. Prior to reaching the condensation temperature of the exhaust gas, the sensor is usually already preheated to a temperature maximally allowed in view of the ceramic damage to be suppressed, so that the sensor is brought to an operating temperature as quickly as possible once the condensation temperature of the exhaust gas has been exceeded. Models, which calculate the exhaust-gas temperature at the sensor installation, are stored in modern engine controls to control this operation, or a heat input into the exhaust system is accumulated. Only if one or also both of these values is/are exceeded will the sensor heating be controlled accordingly, i.e., switched on or increased so as to attain readiness for operation. As an alternative, the exhaust-gas temperature at the critical location in the exhaust duct may also be measured, using a temperature sensor, and read into the engine control device.
The control of the sensor operation, in particular the sensor heating, is problematic in hybrid vehicles. The term hybrid vehicle includes motor vehicles in which at least two drive units which utilize different energy sources in order to provide the power for the vehicle propulsion are combined with one another. The characteristics of an internal combustion engine, which generates kinetic energy through the combustion of gasoline or diesel fuels, and of an electromachine, which converts electrical energy into kinetic energy, complement each other in an especially advantageous manner, which is why modern hybrid vehicles are predominantly equipped with such a combination. Two different hybrid arrangements may be distinguished. In so-called serial or sequential hybrid arrangements the vehicle propulsion is implemented via the electromotor exclusively, whereas the combustion engine, via a separate generator, generates the electric current for charging an energy store feeding the e-motor or for the direct supply of the electromotor. In contrast, parallel hybrid arrangements in which the vehicle drive may be represented both by the combustion engine and the e-motor are preferred these days. In such parallel arrangements, the electromotor, for instance, is typically switched on in operating points having higher vehicle loads, in order to supplement the combustion engine.
In hybrid vehicles, in operating ranges having only low efficiency, especially in idling range, it is basically desired to operate the combustion engine as little as possible, or even not at all, for reasons of fuel efficiency. To this end, it is conventional to equip the hybrid vehicle with an automatic start-stop system, which includes an automatic switchoff that causes an automatic switching off of the combustion engine (or suppresses its renewed switching on) in response to stop conditions, and an automatic switching on, which effects an automatic start of the combustion engine in response to start conditions. In particular, the combustion engine is switched off by the automatic switchoff in standstill phases, i.e., at a vehicle speed of zero. Automatic start-stop systems exploit the fact that hybrid vehicles have considerably stronger electric starter motors than conventional starter motors, which allows a rapid engine start-up, especially in a restart following an automatic stop.
If, in stop operation of the combustion engine, the exhaust gas still remaining in the exhaust system cools to below the condensation temperature and the sensor is kept at its operating temperature in the meantime, then condensation precipitation may occur and the sensor may be acted upon by the condensate, especially liquid water, in a subsequent restart of the engine, so that damage to the sensor may result. On the other hand, if the sensor heating is also reduced or deactivated as soon as the combustion engine is automatically switched off, then the delay at which the sensor regains its operating temperature after a restart of the engine leads to an insufficiently precise engine control and thus to increased emission levels and/or increased fuel consumption.